This invention relates to a strapping band and method for producing same, which can maintain elastic binding force for a long period of time after it has been wound around an article or articles under stretched condition.
In many cases for binding articles by a strapping band, it is desired that the band has a relatively high elasticity and elongation rather than a high tensile strength. Because, an elastic binding force of the band is maintained for a long period of time by the residual stress thereof so that a constant contractive force is applied to the articles wound by the band.
In the event that a plurality of rigid articles such as glass bottles, pipes, tiles, bricks or the like are bound together by a strapping band which has no substantial elasticity, the binding force cannot be maintained since the contractive binding force cannot be applied to the articles by the band.
Further, in the practical binding operation, after winding the band around the article under tension, a heat plate is inserted into a space between the article and the band to join the band together by fusion thereof. Then, the heat plate is removed after fusion of the band. Accordingly, if the band has no substantial elasticity, the tension having been applied to the band at the time of winding will be lost due to the space for the heat plate.
In addition, when a plurality of articles are bound together by a strapping band, the articles tend to become more closely associated with each other by vibration applied thereto. Accordingly, when a strapping band not having a substantial elasticity is used for binding rigid articles, the band will be slackened after the articles have been subjected to vibration. Thus, the binding force having been applied to the articles will be lost.
Conventional plastic strapping bands are made of polypropylene, nylon or other thermoplastic high polymeric material having high crystallinity. Such a high polymeric material is highly elongated in the production process to orient the molecules in the lengthwise direction. The plastic strapping band of this kind has a high tensile strength in the range of about 20-40 kg/mm.sup.2 and high Young's modulus in the range of about 200-700 kg/mm.sup.2 but low elongation percentage of less than about 25%. Accordingly to such a known strapping band, even when the band is stretched under high tension around rigid articles and joined with each other at both ends thereof, the elongation of the band is very small and the residual stress in the band will be eliminated after a slight amount of contraction of the band.
In a practical example of a known polypropylene strapping band having a width of about 15.5 mm (40,000 denier), the band elongates only few percent when wound around articles at a binding force of 50 kg. After binding the articles, if the band contracts for about 2-3% in length from the elongated state, the residual stress in the band is almost eliminated.
As a material having a comparatively small Young's modulus and high elongation as well as high elasticity, rubber is duly considered. However, the rubber material requires vulcanization during formation into a strapping band, thus exhibiting poor productivity in a continuous production process of the strapping band.